In the related art, when increasing the capacity of lead-acid batteries, single batteries or battery packs formed by connecting single batteries in series (a battery pack may have not only portions connected in series but also portions connected in parallel) are connected in parallel as shown in FIG. 6. For example, vehicle lead-acid batteries or the like generally have a large resistance of about 10 mΩ and, even if there are slight voltage differences caused by impedance or contact resistance due to manufacturing variations, the voltage differences are absorbed by this resistance, thus keeping the current bias in the individual batteries small.
In recent years, however, more efficient secondary cells, such as lithium-ion batteries or the like, having lower internal impedance than conventional ones are becoming available, and when batteries are directly connected in parallel in this case, slight differences in impedance or contact resistance due to variability in manufacturing may cause excessively large current to flow at one side of the battery packs. Thus, in recent years, power converters are connected to battery packs, as shown in FIG. 7, and an overcurrent is prevented from flowing in the battery packs by controlling the current that flows in the individual battery packs with the power converters.